The present invention relates to measuring partial discharge and locating of defects in the insulation of conductors and cables. More particularly, the invention relates to detection and locating the occurance of discharges which propagate in the conductor as voltage and current pulses from the location of the defect and in both directions. The conductors and cables in which the problem arises are particularly communication cables, coiled conductors e.g. in machines and transformers or the like.
Investigations concerning the life expectancy of electrical cables resulted in the finding that the life is shortened by discharges due to local defects such as holes or cavities in the insulation. Such discharges will sooner or later destroy the surrounding insulation rendering the cable useless. Thus, a cable has to be tested as to defects which could produce partial discharges.
If an a.c. voltage is applied to a conductor having a defective insulation, electrostatic conditions lead to a partial discharge at that location if the voltage exceeds a particular amplitude which depends on the severity of the defect. As the discharge occurs, current and voltage pulses are produced and they propagate through the cable, particularly for example, along the outer shield. One pulse, for example, runs directly to the end of the cable to which a measuring device is connected. The other pulse, running in the opposite direction travels towards the other end of the conductor, and if that conductor is not terminated by its characteristic impedance, the pulse is reflected and will also arrive at the point of measurement.
The delay between the arrivals of these two pulses depends of course, upon the difference in travel paths. The delay has largest possible value when the location of the pulse producing defect is close to the particular cable end to which the measuring device is connected; the delay is zero if the defect is at the other, pulse-reflecting cable end. It was found, however, that pulses (discharges) of the same intensity may result in different indications as to the said delay. If one considers further that a cable, particularly a long cable, may have more than one defect, one obtains a mixture of superimposed, often oscillatory signals which renders the actual measurement of a defect or defects quite difficult.
The detection of defects by means of the partial discharge method is also difficult because the voltage pulse produced by a discharge in and through an insulation defect has a level that is comparable with the voltage that appears in the cable due to the fact that the cable may act like a radio antenna. Thus, the cable may pick up radio signals or other h.f. signals due to unshielded switching actions in the vicinity. These voltages act as noise with regard to the measurement and the partial discharge voltage may exceed the noise level very little if at all. Therefore, the cable cannot be tested in the field under total disregard to the ambient conditions. Rather, the cable to be tested must be carefully shielded and expensive filter etc. must be employed also. There is, therefore, a need for locating insulation defects in the field.